Updated | German police believe the Pakistani suspect arrested in connection with the truck attack that killed at least 12 people in Berlin was not the driver, and that the perpetrator remains at large, senior security sources told Germany's Die Welt newspaper Tuesday.

"We have the wrong man," a senior police chief told the newspaper. "And therefore a new situation. The true perpetrator is still armed, at large and could do more damage."

Berlin's police chief Klaus Kandt said Tuesday that he was not certain that the arrested suspect was the perpetrator of the truck attack. "As far as I know it is not certain whether he was really the driver," he said.

German federal prosecutor Peter Frank said in a press conference that the suspect in custody "may not have been the perpetrator or belong to the group of perpetrators." He said that the police had not yet confirmed a motive but said that the style of attack points to the modus operandi of radical Islamist groups, with the same method used in the July truck attack in Nice that killed 86, claimed by the Islamic State militant group (ISIS).

German Interior Minister Thomas de Maiziere told reporters Tuesday that the suspect in custody is a man of Pakistani origin who was seeking asylum in Germany as early as December 2015. He said that the suspect denies any involvement in the attack, which left 48 injured, 18 severely.

Berlin police also tweeted, advising residents of the German capital to exercise caution as the perpetrator could still be at large, citing the suspect's denial.

"The temporary arrested suspect denies the offense," Berlin police tweeted. "Therefore we are particularly alert. Please be also alert."

After the Scania truck plowed into the Christmas market at the foot of the Kaiser Wilhelm Memorial Church in central Berlin, the driver fled the scene. Authorities found a passenger, a Polish national, shot dead in the cab of the vehicle. More details about his death began to emerge on Tuesday, providing insight into the incident that German Interior Minister Thomas de Maiziere said he had "no doubt" was an intentional attack.

Ariel Zurawski, the owner of the Polish trucking company to which the vehicle belonged, identified the victim as 37-year-old Lukasz Urban. He said that German authorities had asked him to identify the victim, and said he believed that the assailant had stabbed and shot Urban to death. "His face was swollen and bloodied. It was really clear that he was fighting for his life," Zurawski said, speaking to Polish broadcaster TVN24.

Polish Prime Minister Beata Szydlo said it is "with pain and sadness we received the information that the first victim of this heinous act of violence was a Polish citizen."

No group has claimed responsibility for the incident but both ISIS and Al-Qaeda have previously encouraged the use of car-ramming attacks. After a truck attack in the French city of Nice in July killed 86, ISIS waited two days before releasing a claim of responsibility via its Amaq news agency.

If confirmed that German authorities have arrested the wrong suspect, it would be an embarrassing development for security services and police. In July authorities failed to stop two men, in contact with Islamic State militant group (ISIS) operatives, carrying out attacks in the southern state of Bavaria. ISIS claimed both attacks—one a stabbing attack on a train near Wuerzburg and the other a suicide bombing outside a music festival in Ansbach.

This is a developing story and will be updated as more information becomes available.